Method of Performing Electrochemical Reaction

ABSTRACT

A method of performing an electrochemical reaction in an electrochemical cell comprising electrodes separated by a hydrophilic ion-exchange membrane, comprises conducting the reaction in the presence of an aqueous solution of an electrolyte of which the concentration is controlled.

FIELD OF THE INVENTION

This invention relates to a method of performing an electrochemicalreaction.

BACKGROUND TO THE INVENTION

Electrochemical cells exist in which the electrodes are separated by anion-exchange membrane, for example a solid polymer electrolyte.

WO-A-03/23890 teaches that ion-exchange materials can be produced basedon hydrophilic polymers, i.e. polymers which are inherently able toabsorb and transmit water throughout their molecular structure. Thematerials are particularly relevant to hydrogen-oxygen fuel cells, sinceproduct water can be redistributed, thereby avoiding local flooding ordrying-out of the membrane. There are essentially three ways in which ahydrophilic material can be rendered ionically conducting.

The first way is to form ionically active sites by co-polymerisationfrom a solution of ionically active monomers; this methodology isdescribed in WO-A-03/23890. A strongly anionic or cationic moiety isformed in the resulting polymer, allowing it to function as aanionic-exchange (AE) or cationic-exchange (CE) material respectively.

The second way is to incorporate ionically active sites in the materialby grafting ionically active monomers. An example of such a material isNafion, which becomes ionically conducting when the ionic sites areactivated by hydration in demineralised water.

The third way is by hydration of the hydrophilic material in an acidicor alkaline liquid. Hydrophilic materials such as HEMA (2-hydroxyethylmethacrylate) and MMA-VP (methyl methacrylate-vinylpyrrolidone) possessno intrinsic electrical properties and, if hydrated in deioniseddistilled (DD) water, are good electrical resistors. If, however, thesematerials are hydrated in an acidic or alkaline solution, they becomegood conductors. A limitation of this approach is that conductivitydeclines as the electrolyte solution washes out, the material eventuallybecoming electrically inactive. Such washing out is particularlyproblematic in electrolysers, where reactant water is normally presentin large quantities, and in hydrogen-oxygen fuel cells, where water isproduced.

SUMMARY OF THE INVENTION

The present invention addresses the limitations of the third approachand is based on a realisation that, provided the concentration of theelectrolyte in the membrane can be maintained, a electrically inactivemembrane hydrated with an electrolyte is a viable alternative to otherhydrophilic systems. Similarly, the conductivity of a weakly ionichydrophilic membrane can be augmented by hydrating the membrane in anacidic or alkaline solution. The concentration of the electrolyte can bemaintained by controlling, for example, the extent of hydration of themembrane.

According to the invention, a method of performing an electrochemicalreaction in an electrochemical cell comprising electrodes separated by ahydrophilic ion-exchange membrane, comprises conducting the reaction inthe presence of an aqueous solution of an electrolyte of which theconcentration is controlled.

DESCRIPTION OF PREFERRED EMBODIMENTS

The cell may be in the form of a fuel cell or an electrolyser. In thecase of a fuel cell running on hydrogen and oxygen, water is produced inthe cell reaction. It follows that sufficient quantities of water mustbe removed from the membrane, in order to maintain the concentration ofthe electrolyte. In cases such as this, i.e. where water needs to beremoved from the system, removal is preferably accomplished byevaporation. Evaporation can be achieved by using a Peltier cooled “coldspot” located away from the membrane, at which moisture from the warmcell condenses and is removed. For a cell in which water is consumed,e.g. an electrolyser, the extent of hydration can be controlled bylimiting the amount of water introduced into the cell. The extent ofhydration can be assessed, for example, by monitoring the pH of themembrane during use.

The hydrophilic material may be ionically inactive, i.e. it possesses nointrinsic electrical properties. In this case, ionic activity isconferred to the material by the electrolyte solution. Alternatively,the hydrophilic material may be ionically active, comprising anionic orcationic sites within its structure. Here, the aqueous electrolyteaugments the ionic activity of the hydrophilic material. The inventionhas particular applicability to weakly active materials. Weakly activematerials are those which, when hydrated in DD water, exhibit ionicconductivities which are less than 60%, typically 50%, of that of anindustry standard material such as Nafion 117.

The hydrophilic, material may be obtained using any suitable methodknown in the art. For example, an ionically active material may formedby according to the techniques described in WO-A-03/23890, the contentsof which are incorporated in by reference. The material is preferably ahydrophilic polymer, obtainable by the (co)polymerisation of monomerssuch as methyl methacrylate, N-vinyl-2-pyrrolidone or acrylonitrile, andis preferably cross-linked.

The electrolyte solution may be acidic or alkaline. Examples of suitableelectrolytes include toluenesulphonic acid (TSA), vinylsulphonic acid,acrylamido-(2-methyl)propanesulphonic acid (AMPSA), sodium hydroxide orpotassium hydroxide.

The electrochemical cell is preferably in the form of amembrane-electrode assembly (MEA) or stack of MEAs. MEAs and methods formanufacture are well known in the art. In particular, WO-A-03/23890describes a “one-step” method for the production of MEAs, which involvesforming the membrane by polymerisation in situ.

The following Examples illustrate how the conductivity of a hydrophilicion-exchange membrane can be controlled, as in the present invention.

EXAMPLE 1

A crosslinked hydrophilic polymer was formed by irradiating a mixturecontaining 35% acrylonitrile, 35% N-vinyl-2-pyrrolidone, 5% allylmethacrylate and 25% deionised distilled water with a dose of 2 megaradsin a cobalt 60 source. The resulting solid polymer was divided into twosamples. The first sample was hydrated in water and its electricalconductivity measured using an AC bridge method. The second sample washydrated in a solution of 10% by weight TSA in water, after which itselectrical conductivity was measured in the same test cell as used forthe first sample.

The conductivity of the second sample was shown to be greater than thatof first sample, in the ratio of 5.7:0.07, i.e. 81.4. The conductivityof the second sample was also found to be 36% greater than that of aNafion 117 membrane.

The second sample was then immersed in an excess of DD water for aperiod of 14 days, after which its electrical conductivity was measured.The “residual” conductivity was found to be only 15% of its initialconductivity, demonstrating that substantial loss of TSA had occurred.Subsequent measurements demonstrated a continuing decline inconductivity with time of washing in DD water. Conductivity wasrecovered by the addition of further TSA to the membrane, so that theoriginal pH was restored.

EXAMPLE 2

A crosslinked hydrophilic polymer was formed by irradiating a mixturecontaining 24% methyl methacrylate, 72% N-vinyl-2-pyrrolidone and 4%allylmethacrylate with a dose of 2 megarads in a cobalt 60 source. Theresulting solid polymer was divided into two samples. The first samplewas hydrated in water and its electrical conductivity measured using anAC bridge method. The second sample was hydrated in a solution of 10% byweight TSA in water, after which its electrical conductivity wasmeasured in the same test cell as used for the first sample.

The conductivity of the second sample was shown to be greater than thatof first sample, in the ratio of 47.6:0.05, i.e. 952. The conductivityof the second sample was also found to be 13% greater than that of aNafion 117 membrane.

The second sample was then immersed in an excess of DD water for aperiod of 14 days and after which its electrical conductivity wasmeasured. The “residual” conductivity was found to be only 12% of itsinitial conductivity, demonstrating that substantial loss of TSA hadoccurred. Subsequent measurements demonstrated a continuing decline inconductivity with time of washing in DD water. Conductivity wasrecovered by the addition of further TSA to the membrane, so that theoriginal pH was restored.

1. A method of performing an electrochemical reaction in anelectrochemical cell comprising electrodes separated by a hydrophilicion-exchange membrane, wherein the reaction is conducted in the presenceof an aqueous solution of an electrolyte of which the concentration iscontrolled.
 2. The method according to claim 1, wherein the degree ofhydration of the membrane is controlled.
 3. The method according toclaim 2, wherein the degree of hydration is controlled by removing waterfrom the membrane.
 4. The method according to claim 3, wherein thedegree of hydration is controlled by evaporating water from themembrane.
 5. The method according to claim 2, wherein water is areactant and the input of water into the cell is controlled.
 6. Themethod according to claim 1, wherein the electrolyte is toluenesulphonicacid, vinylsulphonic acid, acrylamido-(2-methyl)propanesulphonic acid,sodium hydroxide or potassium hydroxide.
 7. The method according toclaim 1, wherein the hydrophilic material is ionically inactive.
 8. Themethod according to claim 1, wherein the hydrophilic material isionically active.
 9. The method according to claim 1, wherein thehydrophilic material is a polymeric material.
 10. The method accordingto claim 9, wherein the hydrophilic material is obtainable by thepolymerisation of monomers including methyl methacrylate,N-vinyl-2-pyrrolidone or acrylonitrile.
 11. The method according toclaim 9, wherein the hydrophilic material is cross-linked.
 12. Themethod according to claim 1, wherein the cell is a fuel cell or anelectrolyser.
 13. The method according to claim 1, wherein the cell isin the form of a membrane-electrode assembly (MEA), or a stack of MEAs.14. The method according to claim 1, wherein the concentration iscontrolled by the addition of further electrolyte.